The FileUpload class displays a text box control and a browse button that allow users to select a file on the client and upload it to the Web server. The user specifies the file to upload by entering the full path to the file on the local computer (for example, C:\MyFiles\TestFile.txt) in the text box of the control. Alternately, the user can select the file by clicking the Browse button, and then locating it in the Choose File dialog box.

The FileUpload control does not automatically save a file to the server after the user selects the file to upload. You must explicitly provide a control or mechanism to allow the user to submit the specified file. For example, you can provide a button that the user clicks to upload the file. The code that you write to save the specified file should call the SaveAs method, which saves the contents of a file to a specified path on the server. Typically, the SaveAs method is called in an event-handling method for an event that raises a post back to the server. For example, if you provide a button to submit a file, you could place the code to save the file inside the event-handling method for the click event.

Before calling the SaveAs method to save the file to the server, use the HasFile property to verify that the FileUpload control contains a file. If the HasFile returns true, call the SaveAs method. If it returns false, display a message to the user indicating that the control does not contain a file. Do not check the PostedFile property to determine whether a file to upload exists, because this property contains 0 bytes by default. As a result, even when the FileUpload control is blank, the PostedFile property returns a non-null value.

When you call the SaveAs method, you must specify the full path to the directory in which to save the uploaded file. If you do not explicitly specify a path in your application code, an exception is thrown when a user attempts to upload a file. This behavior helps to keep the files on the server secure by preventing users from being able to write to arbitrary locations in your application's directory structure, as well as preventing access to sensitive root directories.

The SaveAs method writes the uploaded file to the specified directory. Therefore, the ASP.NET application must have write access to the directory on the server. There are two ways that the application can get write access. You can explicitly grant write access to the account under which the application is running, in the directory in which the uploaded files will be saved. Alternatively, you can increase the level of trust that is granted to the ASP.NET application. To get write access to the executing directory for the application, the application must be granted the AspNetHostingPermission object with the trust level set to the AspNetHostingPermissionLevel.Medium value. Increasing the level of trust increases the application's access to resources on the server. Note that this is not a secure approach, because a malicious user who gains control of your application will also be able to run under this higher level of trust. It is a best practice to run an ASP.NET application in the context of a user with the minimum privileges that are required for the application to run. For more information on security in ASP.NET applications, see Basic Security Practices for Web Applications and ASP.NET Trust Levels and Policy Files.

Use the FileName property to get the name of a file on a client to upload using the FileUpload control. The file name that this property returns does not include the path to the file on the client.

The FileContent property gets a Stream object that points to a file to upload. Use this property to access the contents of the file as bytes. For example, you can use the Stream object that is returned by the FileContent property to read the contents of the file as bytes and store them in a byte array. Alternatively, you can use the FileBytes property to retrieve all the bytes in the file.

The PostedFile property gets the underlying HttpPostedFile object for the file to upload. You can use this property to access additional properties on the file. The ContentLength property gets the length of the file. The ContentType property gets the MIME content type of the file. In addition, you can use the PostedFile property to access the FileName property, the InputStream property, and the SaveAs method. However, the same functionality is provided by the FileName property, the FileContent property, and the SaveAs method.

One way to guard against denial of service attacks is to limit the size of the files that can be uploaded using the FileUpload control. You should set a size limit that is appropriate for the types of files that you expect to be uploaded. The default size limit is 4096 KB (4 MB). You can allow larger files to be uploaded by setting the maxRequestLength attribute of the httpRuntime element. To increase the maximum allowable file size for the entire application, set the maxRequestLength attribute in the Web.config file. To increase the maximum allowable file size for a specified page, set the maxRequestLength attribute inside the location element in Web.config. For an example, see location Element (ASP.NET Settings Schema).

When uploading large files, a user might also receive the following error message:

If your users encounter this error message, increase the value of the memoryLimit attribute in the processModel element of the Web.config file for the application. The memoryLimit attribute specifies the maximum amount of memory that a worker process can use. If the worker process exceeds the memoryLimit amount, a new process is created to replace it, and all current requests are reassigned to the new process.

To control whether the file to upload is temporarily stored in memory or on the server while the request is being processed, set the requestLengthDiskThreshold attribute of the httpRuntime element. This attribute allows you to manage the size of the input stream buffer. The default is 256 bytes. The value that you specify should not exceed the value that you specify for the maxRequestLength attribute.

The first code example demonstrates how to create a FileUpload control that saves files to a path that is specified in code.

The second code example demonstrates how to create a FileUpload control that saves files to a specified directory in the file system for the application.

The third code example demonstrates how to create a FileUpload control that saves files to a specified path and limits the size of the file that can be uploaded.

The fourth code example demonstrates how to create a FileUpload control that saves files to a specified path and allows only files with the .doc or .xls extensions to be uploaded.

Caution

These code examples demonstrate the basic syntax for the FileUpload control, but do not demonstrate all the necessary error checking that should be completed prior to saving the file. For a more complete example, see SaveAs.

The following code example demonstrates how to create a FileUpload control that saves files to a path that is specified in code. The SaveAs method is called to save the file to the specified path on the server.

The following code example demonstrates how to create a FileUpload control that saves files to a specified directory in the file system for the application. The HttpRequest.PhysicalApplicationPath property is used to get the physical file system path of the root directory for the currently executing server application. The SaveAs method is called to save the file to the specified path on the server.

The following code example demonstrates how to create a FileUpload control that saves files to a path that is specified in the code. The control limits the size of the file that can be uploaded to 5 MB. The PostedFile property is used to access the underlying ContentLength property and return the size of the file. If the size of the file to upload is less than 5 MB, the SaveAs method is called to save the file to the specified path on the server. In addition to checking for the maximum file size setting in your application code, you can set the maxRequestLength attribute of the httpRuntime element to a maximum allowable size in the configuration file for your application.

The following code example demonstrates how to create a FileUpload control that saves files to a path that is specified in the code. This example allows only files with the .doc or .xls extensions to be uploaded. The Path.GetExtension method is called to return the extension of the file to upload. If the file has a .doc or .xls extension, the SaveAs method is called to save the file to the specified path on the server.